1. Field of the Invention
This invention relates to a process for preparing a finely divided gamma crystal phase quinacridone from a crude quinacridone of an arbitrary crystal phase.
2. Description of the Prior Art
It is well known that quinacridone expressed by the following structural formula (I) possesses three crystal modifications, i.e., alpha, beta and gamma crystal phases. ##STR1## Of the three crystal phases, a gamma crystal phase exhibits an attractive bright shade as well as a high degree of fastness, i.e., resistance to change under various conditions, such as heat, weather, chemicals and solvents, as compared with the alpha and beta crystal phase quinacridones. Therefore, a gamma crystal phase quinacridone is of a wide use.
Many proposals have heretofore been made for the preparation of a gamma crystal phase quinacridone, some examples being the following.
(i) A crude quinacridone is subjected to salt-milling by using an inorganic salt, such as sodium hydroxide, thereby reducing the particle size thereof and subsequently, the salt-milled quinacridone is treated with dimethylformamide (British Pat. No. 828,052).
(ii) A crude quinacridone of an arbitrary crystal phase is ground by using, for example, a ball mill, and then, the finely divided crude quinacridone is heated in a hydrous or anhydrous state, at a temperature of from 80.degree. to 200.degree. C., in the presence of an organic solvent having a solubility of at least 5% by weight in water (Japanese Patent Publication No. 9,272/1964).
(iii) A crude quinacridone is heated together with methanol and potassium hydroxide (Japanese Patent Publication No. 20,073/1964).
(iv) An alpha crystal phase quinacridone is mixed with dimethyl sulfoxide in the presence or absence of boric acid, and the mixture is boiled (Japanese Patent Publication No. 6,098/1965).
(v) An alpha crystal phase quinacridone is heated together with an aliphatic polyamine (NH.sub.2 (R.NH).sub.n H, n=2 or 3) (Japanese Patent Publication No. 1,704/1969).
(vi) An alpha crystal phase quinacridone is heated in alpha-pyrrolidone (British Pat. No. 1,080,394).
(vii) A crude quinacridone is dissolved in a concentrated sulfuric acid and, subsequently, added drop by drop into a lower alcohol, such as methanol, to thereby precipitate a gamma crystal phase quinacridone (British Pat. No. 1,110,997).
The above mentioned conventional processes are not completely satisfactory for the following reasons. The process recited in item (i) requires the step of drying a crude quinacridone prior to the salt-milling thereof. A substantial period of time is needed for the salt-milling and the treatment with dimethylformamide. Moreover, this process recited in item (i) requires the step of washing the quinacridone precipitate for the removal of the inorganic salt therefrom. Thus, a substantial period of time is needed for the completion of the entire process of item (i), and complicated operations are also necessary. In the processes recited in items (ii) through (vi), the resulting quinacridone product particles are not sufficiently minute for use in pigment, and hence, a step of comminuting the product particles or the raw material particles is necessary. The process recited in item (vii) involves the risk that quinacridone will be undesirably sulfonated, and thus, the permmissible operating conditions for dissolution and reprecipitation are particularly restricted. Furthermore, the product particles obtained by the process of item (ii) are generally poor in thermal resistance and weather resistance.
Moreover, all of the conventional processes have a defect such that, in the case where crude qunacridone contains impurities in its particles, it is difficult to remove the purities in the course of manufacturing gamma crystal phase quinacridone. Therefore, it is generally required to purify crude quinacridone prior to the use thereof in the manufacture of the gamma crystal phase quinacridone.